Medical School · Year 1 · Anatomy Pelvis Head Neck · includes a quiz and discussion video
Lecture 1: Pelvic Bones and Ligaments
Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck
Learning Objectives
By the end of this lecture, students will be able to:
- Identify the bones comprising the bony pelvis
- Describe the features of the hip bone, sacrum, and coccyx
- Distinguish between the greater and lesser pelvis
- Identify the major ligaments of the pelvis
- Compare male and female pelvic anatomy
- Describe the clinical significance of pelvic measurements
Overview of the Bony Pelvis
The bony pelvis forms a ring-like structure composed of two hip bones (ossa coxae) articulating anteriorly at the pubic symphysis and posteriorly with the sacrum at the sacroiliac joints. The sacrum continues inferiorly as the coccyx, completing the posterior wall. This osseous basin serves several critical functions: it transmits the weight of the trunk to the lower limbs through the hip joints, protects the pelvic viscera including portions of the gastrointestinal and urogenital systems, provides extensive attachment sites for muscles of the trunk, abdomen, and lower limb, and in females, forms the birth canal through which the fetus passes during delivery.
The joints of the pelvis permit minimal movement, emphasizing stability over mobility. The pubic symphysis represents a secondary cartilaginous joint (symphysis) with a fibrocartilaginous disc, allowing only slight compression and rotation during weight-bearing and childbirth. The sacroiliac joint combines synovial articulation along its anterior and inferior portions with fibrous syndesmosis posteriorly, where powerful interosseous ligaments bind the bones together. This dual nature provides the stability needed to transmit forces while permitting the subtle movements necessary during locomotion. The sacrococcygeal joint, another secondary cartilaginous joint, allows limited flexion and extension of the coccyx, particularly during defecation.
<image>Panel A: Anterior view of the complete bony pelvis showing the two hip bones forming the lateral walls. Panel B: Pubic symphysis at the anterior midline with white fibrocartilaginous disc. Panel C: Wedge-shaped sacrum occupying the posterior midline with sacroiliac joint articulations. Panel D: Small triangular coccyx hanging inferiorly from the sacral apex with labeled components and joints.</image>
The Hip Bone (Os Coxae)
Development and Component Bones
The hip bone develops from three separate ossification centers that fuse at the acetabulum during adolescence. The ilium forms the superior portion, contributing the broad wing (ala) that flares superolaterally and the body that participates in the acetabulum. The ischium forms the posteroinferior portion, providing the sturdy bone upon which we sit. The pubis forms the anteroinferior component, creating the anterior pelvic wall. These three bones meet and fuse at the acetabulum, the cup-shaped socket for the hip joint, with complete fusion typically occurring between ages 15 and 25.
The Ilium
The ilium's superior portion forms the broad, curved ala (wing) that provides extensive surface area for muscle attachment. The iliac crest runs along the superior border as a palpable landmark extending from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). The ASIS serves as the attachment point for the inguinal ligament and sartorius muscle, while the PSIS marks the S2 vertebral level and overlies the sacroiliac joint. Below these lie the anterior inferior iliac spine (AIIS), providing origin for the rectus femoris, and the posterior inferior iliac spine (PIIS). The tubercle of the iliac crest, marking the widest point, lies at the L5 vertebral level and provides attachment for the iliotibial tract.
The lateral (gluteal) surface displays three curved lines—posterior, anterior, and inferior gluteal lines—that demarcate the origins of the three gluteal muscles. The medial surface presents the smooth, concave iliac fossa (origin of iliacus muscle) and the sacropelvic surface, which includes the auricular surface for sacroiliac articulation and the iliac tuberosity for ligamentous attachments.
The Ischium
The ischium contributes the posteroinferior part of the acetabulum through its body. The ischial tuberosity, a rough prominence at the junction of the body and ramus, bears the body's weight during sitting and provides origin for the hamstring muscles. The ischial spine projects into the pelvic cavity, providing attachment for the sacrospinous ligament and the levator ani muscle. The greater sciatic notch lies above the spine, while the lesser sciatic notch lies below. The ischial ramus extends anteriorly and superiorly to join the inferior pubic ramus, together forming the ischiopubic ramus.
The Pubis
The pubis consists of a body that forms the anterior acetabulum and meets its counterpart at the pubic symphysis. The pubic tubercle, projecting from the upper body, marks the medial attachment of the inguinal ligament and serves as a crucial landmark in hernia examination. The pubic crest runs medially from the tubercle along the superior border. The superior pubic ramus extends laterally to the acetabulum, while the inferior pubic ramus descends to join the ischial ramus. The pecten pubis (pectineal line) forms the sharp superior edge of the superior ramus, continuing the pelvic brim.
<image>Panel A: Lateral view of hip bone showing ilium (light blue), ischium (green), and pubis (yellow) meeting at the acetabulum. Panel B: Labeled iliac crest, ASIS, PSIS, and gluteal lines on the lateral surface. Panel C: Ischial tuberosity inferiorly with ischial spine and pubic tubercle anteriorly. Panel D: Medial view revealing iliac fossa, auricular surface for sacroiliac articulation, and obturator foramen.</image>
The Acetabulum
The acetabulum forms a deep, cup-shaped cavity on the lateral aspect of the hip bone, oriented laterally, inferiorly, and anteriorly to receive the femoral head. All three component bones contribute to its formation, meeting at a Y-shaped growth plate (triradiate cartilage) that fuses in adolescence.
The lunate surface comprises the articular portion, a crescent-shaped area covered with hyaline cartilage that articulates with the femoral head. The acetabular fossa occupies the central floor as a non-articular depression filled with fat pad (pulvinar) and the ligament of the femoral head (ligamentum teres). The acetabular notch interrupts the inferior rim, transmitting vessels to the femoral head and providing passage for the transverse acetabular ligament.
The fibrocartilaginous acetabular labrum attaches to the bony rim, deepening the socket and creating a seal around the femoral head that maintains synovial fluid in the joint and provides suction stability. The transverse acetabular ligament bridges the acetabular notch, converting it into a foramen.
Acetabular orientation varies slightly among individuals but averages approximately 15-20 degrees of anteversion (forward facing of the opening) and 45 degrees of inclination (angle from horizontal). These parameters influence hip joint mechanics and may predispose to dysplasia if abnormal.
<image>Panel A: Lateral view of right acetabulum with crescent-shaped lunate surface in ivory representing articular cartilage. Panel B: Central acetabular fossa in pink (non-articular, fat-filled) with acetabular notch as inferior rim gap. Panel C: Fibrocartilaginous labrum as blue ring with transverse acetabular ligament spanning the notch. Panel D: Diagrams illustrating anteversion (superior view) and inclination (lateral view with angle measurement).</image>
The Sacrum
The sacrum forms through fusion of five sacral vertebrae (S1-S5) into a single wedge-shaped bone that constitutes the posterior wall of the pelvis. Its triangular outline, with base superiorly and apex inferiorly, allows it to function as a keystone, transmitting body weight from the spine to the hip bones through the sacroiliac joints.
The anterior (pelvic) surface faces the pelvic cavity and presents several distinctive features. The sacral promontory forms a prominent anterior projection of the S1 vertebral body, marking the posterior limit of the pelvic inlet—a crucial obstetric landmark. Four transverse ridges mark the lines of fusion between vertebral bodies. Four pairs of anterior (pelvic) sacral foramina transmit the ventral rami of the sacral spinal nerves, which contribute to the sacral plexus.
The posterior surface displays features derived from the fused vertebral elements. The median sacral crest represents the fused spinous processes as a midline ridge. The intermediate sacral crests, lateral to this, represent fused articular processes. The lateral sacral crests mark fused transverse processes. Four pairs of posterior sacral foramina transmit the dorsal rami of the sacral nerves. The sacral hiatus appears at the S4-S5 level as a gap where the laminae fail to fuse, providing access to the sacral canal for caudal epidural anesthesia. The sacral cornua flank this hiatus as small projections that articulate with the coccygeal cornua.
The lateral surface features the auricular surface (ear-shaped) for articulation with the ilium and the sacral tuberosity posterior to this for attachment of the interosseous sacroiliac ligament.
The sacral canal continues the vertebral canal, containing the cauda equina nerve roots, filum terminale, and spinal meninges as they descend toward their exit points.
<image>Panel A: Anterior view of sacrum showing broad base with prominent sacral promontory projecting forward. Panel B: Four transverse ridges marking vertebral fusion with four pairs of anterior sacral foramina. Panel C: Posterior view revealing median, intermediate, and lateral sacral crests with posterior foramina. Panel D: Sacral hiatus at inferior end with sacral cornua and lateral view inset showing auricular surface.</image>
The Coccyx
The coccyx represents the vestigial tail, formed by fusion of three to five (usually four) coccygeal vertebrae into a small triangular bone articulating with the sacral apex. The base of the coccyx articulates with S5 at the sacrococcygeal joint, a secondary cartilaginous joint permitting limited flexion and extension. Coccygeal cornua project superiorly from the first coccygeal segment to articulate with the sacral cornua. Small transverse processes also extend from the first segment.
Despite its diminutive size, the coccyx provides important attachments for the gluteus maximus posteriorly, the coccygeus muscle laterally, and the levator ani muscle anterolaterally. The anococcygeal ligament (raphe) extends from the coccyx to the external anal sphincter, contributing to perineal support.
<image>Panel A: Anterior view of coccyx showing small triangular bone with base articulating with S5. Panel B: Coccygeal cornua projecting superiorly with transverse processes extending laterally. Panel C: Posterior view with tapering apex pointing inferiorly. Panel D: Arrows indicating gluteus maximus, coccygeus, and levator ani muscle attachments with anococcygeal ligament.</image>
Division of the Pelvis
The pelvic brim (linea terminalis) divides the pelvis into greater (false) and lesser (true) pelvis. This continuous line comprises several structures: the sacral promontory posteriorly, the arcuate line running along the internal surface of the ilium, the pecten pubis (pectineal line) on the superior pubic ramus, the pubic crest, and the superior margin of the pubic symphysis anteriorly.
The greater pelvis (false pelvis) lies above the pelvic brim and technically belongs to the abdominal cavity. Its walls are formed by the iliac alae, and it contains lower abdominal viscera including portions of the small intestine, cecum, appendix, and sigmoid colon. The iliacus muscle lines the iliac fossa within this region.
The lesser pelvis (true pelvis) lies below the pelvic brim and contains the true pelvic organs: the rectum, urinary bladder, and reproductive organs. In females, this space forms the birth canal through which the fetus must navigate during delivery. The lesser pelvis is bounded by the sacrum and coccyx posteriorly, the ischium and pubis anterolaterally, and the pelvic floor inferiorly.
<image>Panel A: Sagittal section through female pelvis with pelvic brim highlighted as continuous colored line. Panel B: Greater pelvis (light blue) above the brim containing small bowel loops. Panel C: Lesser pelvis (light pink) below containing bladder anteriorly, uterus centrally, and rectum posteriorly. Panel D: Pelvic floor muscles closing the inferior aperture with labeled anatomical landmarks.</image>
Pelvic Ligaments
Sacroiliac Ligaments
The sacroiliac joint, critical for weight transmission, is reinforced by some of the body's strongest ligaments. The anterior sacroiliac ligament forms a thin sheet covering the anterior joint surface, reinforcing the joint capsule. The interosseous sacroiliac ligament, the strongest of the group, fills the space between the sacral and iliac tuberosities, forming the primary bond between these bones. The posterior sacroiliac ligament consists of short fibers (between the intermediate sacral crest and iliac tuberosity) and long fibers (from the lateral sacral crest to the PSIS), resisting downward movement of the sacrum.
Accessory Ligaments
The iliolumbar ligament extends from the L5 transverse process to the iliac crest, stabilizing the lumbosacral junction and limiting excessive movement at L5-S1. The sacrotuberous ligament is a powerful band extending from the PSIS, lateral sacrum, and coccyx to the ischial tuberosity. It forms the posterolateral boundary of the lesser sciatic foramen and resists rotation of the sacrum (nutation) under load.
The sacrospinous ligament runs from the lateral sacrum and coccyx to the ischial spine. This triangular ligament lies deep to the sacrotuberous ligament and serves the crucial function of converting the greater and lesser sciatic notches into foramina—passages for important neurovascular structures between the pelvis and gluteal region.
Pubic Ligaments
The pubic symphysis is reinforced by the superior pubic ligament above and the inferior pubic ligament (arcuate pubic ligament) below. Between the opposing pubic bodies lies the interpubic disc, a fibrocartilaginous structure that may contain a small fluid-filled cavity. During pregnancy, increased relaxin hormone softens these ligaments, allowing increased mobility to facilitate delivery.
<image>Panel A: Posterior view of pelvis showing three sacroiliac ligaments in layers (anterior, interosseous, posterior). Panel B: Iliolumbar ligament extending from L5 to iliac crest. Panel C: Sacrotuberous ligament from sacrum to ischial tuberosity with sacrospinous ligament to ischial spine and sciatic foramina labeled. Panel D: Inset of pubic symphysis showing superior, inferior (arcuate), and interpubic structures.</image>
Sciatic Foramina
The sacrospinous and sacrotuberous ligaments convert the sciatic notches of the hip bone into foramina, creating passages between the pelvic cavity and the gluteal region.
The greater sciatic foramen is bounded by the greater sciatic notch (of the ilium) superiorly, the sacrospinous ligament inferiorly, and the sacrotuberous ligament posterolaterally. The piriformis muscle passes through this foramen, dividing it into suprapiriform and infrapiriform compartments. Structures passing above the piriformis include the superior gluteal nerve and vessels. Structures passing below include the inferior gluteal nerve and vessels, sciatic nerve, posterior femoral cutaneous nerve, pudendal nerve, internal pudendal vessels, and nerve to obturator internus.
The lesser sciatic foramen is bounded by the lesser sciatic notch superiorly, the sacrospinous ligament above, and the sacrotuberous ligament below. Three structures pass through this foramen: the tendon of obturator internus, the nerve to obturator internus, and the pudendal nerve and internal pudendal vessels (which exit the pelvis through the greater foramen, cross the ischial spine, and re-enter the perineum through the lesser foramen to reach the pudendal canal).
<image>Panel A: Lateral view of pelvis showing greater sciatic foramen with piriformis muscle (red) and structures passing above (superior gluteal nerve and vessels). Panel B: Structures below piriformis including sciatic nerve (yellow), inferior gluteal bundle, and pudendal bundle (purple). Panel C: Lesser sciatic foramen inferiorly with obturator internus tendon passing through. Panel D: Arrows showing pudendal nerve path through greater foramen, across ischial spine (red dot), and into lesser foramen.</image>
Obturator Foramen
The obturator foramen is the large opening in the hip bone bounded by the bodies and rami of the pubis and ischium. In life, the obturator membrane, a fibrous sheet, covers nearly the entire foramen, leaving only a small gap at the superolateral aspect called the obturator canal.
The obturator canal, approximately 2.5 centimeters long, provides passage from the pelvis to the medial thigh for the obturator nerve and obturator vessels. An obturator hernia, more common in elderly women, may protrude through this canal, potentially compressing the obturator nerve and causing pain along its cutaneous distribution on the medial thigh (Howship-Romberg sign).
<image>Panel A: Anterolateral view of hip bone showing obturator foramen covered by obturator membrane (translucent blue). Panel B: Obturator canal at superolateral corner as gap in the membrane. Panel C: Obturator nerve (yellow) and vessels (red artery, blue vein) passing through the canal. Panel D: Labeled foramen boundaries with pubic body and rami superiorly, ischial body and ramus posteriorly.</image>
Sexual Dimorphism of the Pelvis
The pelvis demonstrates marked sexual dimorphism reflecting functional differences, particularly the female adaptation for childbirth.
The female pelvis is generally wider and shallower than the male pelvis. The pelvic inlet in females is oval or nearly round, whereas in males it appears heart-shaped due to the more prominent sacral promontory. The pelvic outlet is larger in females, facilitating delivery. The subpubic angle, measured between the inferior pubic rami, exceeds 80 degrees in females (obtuse), compared to less than 70 degrees in males (acute). The female sacrum is shorter, wider, and less curved than the male sacrum. The ischial spines in females are further apart, providing more room in the pelvic cavity. The greater sciatic notch is wide (greater than 90 degrees) in females but narrow (less than 70 degrees) in males. The acetabulum in females is smaller and faces more anteriorly, while in males it is larger and faces more laterally.
The Caldwell-Moloy classification categorizes female pelves into four types based on inlet shape. The gynecoid pelvis, with a round inlet, occurs in approximately 50% of women and is most favorable for vaginal delivery. The android pelvis (heart or wedge-shaped inlet, 20%) resembles the male pattern and may cause labor difficulties. The anthropoid pelvis (oval inlet with anteroposterior elongation, 25%) often allows delivery but may require assisted rotation. The platypelloid pelvis (transversely flattened oval, 5%) is the least common and may impede fetal descent.
<image>Panel A: Female pelvis (anterior view) showing wider, shallower shape with rounded inlet and obtuse subpubic angle. Panel B: Male pelvis (anterior view) showing narrower, deeper shape with heart-shaped inlet and acute subpubic angle. Panel C: Comparison of greater sciatic notches (wide in female, narrow in male). Panel D: Caldwell-Moloy classification showing gynecoid (round, favorable), android (heart-shaped), anthropoid (oval), and platypelloid (transverse oval) pelvic types.</image>
Pelvic Measurements (Obstetric)
Obstetric pelvimetry assesses pelvic dimensions critical for predicting the feasibility of vaginal delivery. At the pelvic inlet, several diameters are measured. The true conjugate (anatomical conjugate) extends from the sacral promontory to the closest point on the posterior pubic symphysis, measuring approximately 11 centimeters. The obstetric conjugate, the most important diameter, runs from the promontory to the most projecting point on the posterior symphysis, measuring approximately 10 centimeters. This represents the narrowest fixed anteroposterior diameter that the fetal head must negotiate. The diagonal conjugate, from the promontory to the inferior border of the symphysis (approximately 12.5 cm), is the only conjugate measurable clinically on vaginal examination; the obstetric conjugate is estimated by subtracting 1.5-2 cm from this measurement. The transverse diameter, the widest measurement at the inlet (approximately 13 cm), runs between the most lateral points on the pelvic brim. The oblique diameter extends from one sacroiliac joint to the opposite iliopubic eminence, measuring approximately 12.5 centimeters.
At the pelvic outlet, the anteroposterior diameter from the tip of the coccyx to the inferior symphysis measures approximately 9.5 centimeters but can increase with coccygeal extension during delivery. The transverse (bi-ischial) diameter between the ischial tuberosities measures approximately 11 centimeters.
An obstetric conjugate less than 10 centimeters may indicate a contracted pelvis, potentially necessitating cesarean delivery. Modern practice relies more on clinical judgment and labor monitoring than on pelvimetry alone, but understanding these measurements remains important for recognizing obstructed labor.
<image>Panel A: Superior view of pelvic inlet showing true, obstetric, and diagonal conjugates as colored lines with measurements. Panel B: Transverse and oblique diameters of the inlet with centimeter labels. Panel C: Lateral view showing conjugate relationships to symphysis and inferior view of outlet diameters. Panel D: Clinical inset demonstrating vaginal examination technique for measuring diagonal conjugate.</image>
Clinical Correlations
Pelvic fractures typically result from high-energy trauma such as motor vehicle accidents and falls from height. The pelvic ring requires disruption at two points to become unstable, often involving both anterior (pubic) and posterior (sacroiliac) components. The "open-book" fracture results from anteroposterior compression that disrupts the pubic symphysis and anterior sacroiliac ligaments, causing the pelvis to hinge open like a book on the posterior ligaments. This injury pattern carries significant hemorrhage risk due to disruption of the presacral venous plexus and iliac vessels. Emergency stabilization with pelvic binders reduces pelvic volume and may tamponade bleeding.
Sacroiliac joint dysfunction represents a common cause of low back and buttock pain, resulting from either excessive or insufficient joint mobility. Inflammation of the sacroiliac joint (sacroiliitis) characterizes seronegative spondyloarthropathies, particularly ankylosing spondylitis, where progressive ossification may eventually fuse the joint.
Coccygodynia, pain localized to the coccyx, commonly follows direct trauma such as a fall onto the buttocks. The pain worsens with sitting and transitional movements. Conservative management succeeds in most cases, though refractory cases may rarely require coccygectomy.
Pubic symphysis dysfunction occurs most commonly during pregnancy, when hormonal changes (particularly relaxin) soften the ligaments, allowing increased mobility and potentially painful instability. Symphyseal separation exceeding 10 millimeters may cause significant disability requiring supportive measures.
<image>Panel A: Open-book pelvic fracture with widened pubic symphysis and external rotation of hemipelves with force arrows. Panel B: Sacroiliitis with inflammation at sacroiliac joint (red) and early ankylosing spondylitis bone spurs. Panel C: Coccygeal injury mechanism from fall onto buttocks with fracture/dislocation indicated. Panel D: Pubic symphysis separation during pregnancy with measurement scale and gravid uterus above.</image>
Summary
The bony pelvis consists of two hip bones articulating anteriorly at the pubic symphysis and posteriorly with the sacrum at the sacroiliac joints, with the coccyx continuing inferiorly. Each hip bone forms from fusion of the ilium, ischium, and pubis at the acetabulum. The pelvic brim (linea terminalis) divides the greater (false) pelvis above from the lesser (true) pelvis below. The major ligaments—sacroiliac, sacrotuberous, and sacrospinous—stabilize the joints and, in the case of the latter two, convert the sciatic notches into the greater and lesser sciatic foramina. The female pelvis is wider with a larger outlet and more obtuse subpubic angle compared to the narrower, deeper male pelvis. Pelvic measurements, particularly the obstetric conjugate at the inlet, are critical for obstetric assessment and delivery planning.
Key Terms
Os coxae: The hip bone, formed by fusion of the ilium (superior), ischium (posteroinferior), and pubis (anteroinferior) at the acetabulum.
Pelvic brim (linea terminalis): The continuous line of structures separating the greater pelvis above from the lesser pelvis below, comprising the sacral promontory, arcuate line, pecten pubis, pubic crest, and pubic symphysis.
Sacral promontory: The anterior projection of the S1 vertebral body, forming the posterior landmark of the pelvic inlet and a critical reference point in obstetric measurements.
Sacrotuberous ligament: The strong ligament extending from the PSIS and lateral sacrum to the ischial tuberosity, forming the posterior boundary of the lesser sciatic foramen and resisting sacral rotation.
Obstetric conjugate: The shortest anteroposterior diameter of the pelvic inlet, measuring approximately 10 centimeters from the sacral promontory to the most projecting point of the pubic symphysis.
Gynecoid pelvis: The most common female pelvic type (approximately 50%), characterized by a round inlet and considered most favorable for vaginal delivery.
This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.
